The present invention relates to a device and method to prevent hip fractures.
The femur is the longest and largest bone in the human body. The femur forms part of the hip at one end and part of the knee at the other end. FIG. 1 is a front view of the upper portion of a femur, illustrating the various parts or areas of the femur 40, including the femoral head 42, the femoral neck 44, and the greater trochanter 46.
The femoral head 42 is generally globular and is directed upward, medialward, and a little forward, with the greater part of its convexity being above and in front. See Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000.
The femoral neck 44 is a truncated conical process of bone, connecting the femoral head 42 with the rest of the femur 40, and forming with the latter a wide angle opening medialward. Id. The femoral neck 44 is contracted in the middle and is broader laterally than medially. Id. The upper or superior border 45 of the neck 44 is short and thick, and ends laterally at the greater trochanter 46. Id. The inferior border, long and narrow, curves a little backward, to end at the lesser trochanter. Id.
The greater trochanter 46 is a large, irregular, quadrilateral eminence, situated at the junction of the neck 44 with the upper part of the femur 40. Id. The greater trochanter 46 has two surfaces and four borders. Id. The lateral surface, quadrilateral in form, is broad, rough, convex, and marked by a diagonal impression, which extends from the postero-superior to the antero-inferior angle. Id. The medial surface, of much less extent than the lateral, presents at its base a deep depression, the trochanteric fossa (digital fossa). Id. The superior border is free; it is thick and irregular, and marked near the center by an impression. Id. The inferior border corresponds to the line of junction of the base of the trochanter with the lateral surface of the body; it is marked by a rough, prominent, slightly curved ridge. Id. The anterior border is prominent and somewhat irregular. Id. The posterior border is very prominent and appears as a free, rounded edge, which bounds the back part of the trochanteric fossa. Id.
The femoral shaft 47 is generally cylindrical. Id. The femoral shaft 47 is slightly arched, so as to be convex in front, and concave behind, where it is strengthened by a prominent longitudinal ridge, the linea aspera. Id.
FIG. 1 also illustrates the axis of the normal load-bearing vector 48, i.e., the axis upon which loads act during walking, standing, and other activities of daily living. FIG. 1 further illustrates the general orientation of the longitudinal axis 50 of the femoral neck 44, along with the long axis 52 of the femoral shaft 47 of the femur 40.
Referring now to FIGS. 2-5, hip fractures commonly result from a fall to the side in which impact with the ground occurs over the greater trochanter 46 of the lateral femur 40. During a hip fracture, the impact from a fall to the side (as best shown in FIG. 2) results in a three-point bending of the femur 40, including a “reverse bending” load on the femoral neck 44 with the upper (or superior) border or side 45 of the femoral neck 44 developing a compressive stress and the lower (or inferior) border or side 49 of the femoral neck 44 developing a tensile stress. It is believed that the weaker upper border or side 45 of the femoral neck 44 most likely fails or cracks first in compression, as indicated by reference numeral 56 in FIG. 3. After the initial failure, the crack propagates across the entire femoral neck 44, including the stronger lower side 45 where the predominant load is tension/bending, as shown in FIG. 4. Finally, depending on the direction of the crack propagation, this culminates in a hip fracture, either as a neck fracture 58 or an intertrochanteric fracture 60, as shown in FIG. 5.
For further information about the mechanics of hip fracture, see Turner, C H. The Biomechanics of Hip Fracture. Lancet. 2005 July 9-15;366(9480):98-9. See also Mansek, Sarah et al. Failure in Femoral Neck Fractures Initiates in the Superolateral Cortex: Evidence from High Speed Video of Simulated Fracture. Poster No. 943, 54th Annual Meeting of the Orthopaedic Research Society (2008). Each of these articles is incorporated herein by reference.
Medical treatment is available for a hip fracture, often in the form of a screw that is inserted into the femur, passing across the fracture along the longitudinal axis 50 of the femoral neck 44 at an approximately 45° angle with respect to the long axis 52 of the femoral shaft 47. However, there is a need for preventing hip fractures, and, more particularly, for preventing fractures along and in the region near the junction between the femoral neck 44 and the greater trochanter 46. That being said, there is a common fear that putting a metal (e.g., titanium) implant in an otherwise normal (i.e., not fractured) femur to prevent hip fractures will result in bone loss around the implant due to the relative unloading of the bone from the load-sharing nature of the stiffer metal. This phenomenon is commonly referred to as “stress shielding.”
Stress shielding refers to a reduction in bone density (osteopenia) as a result of removal of normal stress from the bone by an implant (for instance, the femoral component of a hip prosthesis). According to Wolff's Law, osteopenia occurs because a bone in a healthy person or animal will remodel in response to the loads it is placed under. Therefore, if the loading on a bone decreases, the bone will become less dense and weaker because there is no stimulus for continued remodeling that is required to maintain bone mass.
Related to the concept of stress shielding is the phenomenon that the skeleton is a self-optimizing structure. Bone material in highly stressed or strained regions is preserved while bone in the low stress and strain regions is diminished by the natural remodeling process. In the hip, the bone in the inferior region of the femoral neck, otherwise referred to as the calcar region, is very dense due to the constant state of high stress and strain due to the load produced by standing and walking. Conversely, the bone in the superior region of the femoral neck, and, in particular, in the region near the junction of the superior femoral neck and the greater trochanter, becomes increasingly less dense over time due to the lack of direct loading during walking, standing, and other activities of daily living. Thus, the bone region that is the subject of greatest interest in the present application is continually being diminished in quality by the natural processes of bone remodeling. The normal bone remodeling process continually removes bone from the region of the superior femoral neck because standing, walking, or other daily activity does not generate high loads in this area. The normal load-bearing vector due to walking or other normal daily activity passes from the superior surface of the femoral head through the head to the calcar region of the proximal medial femoral shaft cortex, which is shown as the normal load-bearing vector 48 in FIG. 1. It is for this reason that natural or pharmacologic methods to augment the strength of bone often show poor results in preventing hip fractures as opposed to other fractures in other regions of the body.
Thus, there remains a need for a device and method to prevent hip fractures along and in the region near the femoral neck without causing stress shielding.